Unfortunately authors have the right to remove their files as and when they choose as themeforest does not own any form of copyright on them. So technically the only way you would be entitled to a refund is if the theme is broken or does not work as advertised.

You can try submitting a ticket to support at envato (bear in mind replies can take time due to the number of members and time zones etc) or if you remember the author you can try contacting them as it maybe a temporary removal.

well, the theme doesn’t work as advertised. lots of issues with it; they updated it twice and didn’t really fix the issues and now its gone. Its $80!! Lots of people had issues with it. Its only been up 1.5-2 weeks. Feels like a bait and run. They just collected over $4k from the 60+ sales and pulled the theme.